Field of the Invention
The present invention relates to an I/O module and a process control system.
Priority is claimed on Japanese Patent Application No. 2013-243536, filed Nov. 26, 2013, the contents of which are incorporated herein by reference.
Description of Related Art
A process control system constructed in a plant or a factory or the like generally has a constitution in which on-site devices called field devices (measuring devices, and actuators), a controller for controlling the operations of field devices, and a host device for managing and controlling the field devices and the controller are connected one another via a communication means. In such process control system, the controller collects process values obtained by the field devices (for example, measured values such as pressure, temperature, and flow rate), and controls the field devices depending on the collected process values under the control of host device.
It is often the case that the process control system includes an operation monitoring terminal to be operated by an operator in a plant or the like and a device management apparatus for determining the normality of process control system. Specifically, the operation monitoring terminal is for conveying the behavior of elements (the field device and the controller), which constitute the process control system, to the operator, and is for controlling the controller based on the instructions from the operator. The device management apparatus is for collecting information indicating the status of each element, which constitutes the process control system, and their setting information to determine the normality of each element, and is for, if needed, conveying the information indicating the normality to an operator or a maintenance worker.
Each of Japanese Patent No. 3641137, Japanese Laid-open Patent Publication No. 2013-73503, and Japanese Laid-open Patent Publication No. 2013-152612 discloses an example of a monitoring control system for monitoring and controlling various plants. Specifically, Japanese Patent No. 3641137 discloses an example of distributed control system, which includes a plurality of local controllers and is for independently controlling each local controller. Japanese Laid-open Patent Publication No. 2013-73503 discloses an example of distributed monitoring control device capable of separating a programming of control logic in each control device and an engineering of process input and output. Japanese Laid-open Patent Publication No. 2013-152612 discloses an example of plant monitoring control system capable of easily changing monitoring images of monitoring device depending on the operation patterns.
The communication performed in the process control system is roughly divided into a communication for controlling the process (hereinafter, referred to as a process control communication) and a communication for determining the normality of process control system (hereinafter, referred to as a normality-determination-communication). The process control communication is performed at a constant frequency between the controller and the field device. Since it is necessary to control process values in real time, the priority of process control communication is set to be higher than that of the normality-determination-communication. On the other hand, the normality-determination-communication is performed in a polling method between the device management apparatus and the field device via the controller. The priority of normality understanding communication is set to be lower than that of the process control communication not to affect the process control communication.
The communication interval of normality-determination-communication is required to be wider than that of the process control communication not to affect the process control. Therefore, there are some cases where it takes a long time to collect data required for the determination of the normality of process control system using the device control apparatus. For example, if the number of elements, which constitute the process control system, becomes approximately several thousands to several tens of thousands, there are some cases where it takes more than an hour to communicate with all of these elements in a polling method. Thereby, when some abnormalities or changes of the elements, which constitute the process control system, are occurred, there are some cases where the recognition of the abnormalities or changes by the device management apparatus is significantly delayed.
The normality-determination-communication in the polling method is performed regardless of whether the abnormalities or changes of the elements occur, which constitute the process control system. Therefore, there are some cases where the communication is performed despite the absence of the abnormalities or changes, thereby the communication affects the communication system. For example, in the large-sized process control system in which a number of field devices are installed, there are some cases where the communication capacity becomes tight due to the occurrence of unnecessary communication.
In recent years, a field device has had intelligent functions, and it is often the case that the field device has, for example, a self-diagnosis function for diagnosing its own status. The field device, which has such functions, is capable of autonomously notifying a host device of the self-diagnosis results and the information indicating the abnormalities and changes which have occurred in the field device. However, such autonomous notification causes an interruption at a non-constant frequency for the controller. Therefore, there are some cases where the notification increases the load of controller and affects the process control communication.